How do you validate file format based on file signature (magic number) instead of just file extensio

Question

ICSM Computer · Answer

Validating a file's format using its signature (magic number) is much more reliable than checking the file extension, because it inspects the actual
byte content of the file header.
What is a File Signature / Magic Number?
A magic number is a unique sequence of bytes at the beginning of a file that identifies its type.
File Type
Signature (Hex)
ASCII Example
PDF
25 50 44 46
%PDF
PNG
89 50 4E 47 0D 0A 1A 0A
.PNG....
JPG
FF D8 FF
and 
ZIP
50 4B 03 04
PK..
DOCX
50 4B 03 04
ZIP container
How to Read and Validate Signature in C#
using System;
using System.IO;
public class FileSignatureValidator
{
    public static bool IsPdf(string filePath)
    {
        byte[] pdfSignature = { 0x25, 0x50, 0x44, 0x46 }; // %PDF
        return HasSignature(filePath, pdfSignature);
    }
    public static bool IsPng(string filePath)
    {
        byte[] pngSignature = { 0x89, 0x50, 0x4E, 0x47 };
        return HasSignature(filePath, pngSignature);
    }
    public static bool HasSignature(string filePath, byte[] expectedSignature)
    {
        if (!File.Exists(filePath))
            return false;
        using (var stream = File.OpenRead(filePath))
        {
            byte[] buffer = new byte[expectedSignature.Length];
            int read = stream.Read(buffer, 0, buffer.Length);
            if (read != expectedSignature.Length)
                return false;
            for (int i = 0; i andlt; expectedSignature.Length; i++)
            {
                if (buffer[i] != expectedSignature[i])
                    return false;
            }
            return true;
        }
    }
}
Usage:
if (FileSignatureValidator.IsPdf("document.pdf"))
    Console.WriteLine("This is a PDF file.");
else
    Console.WriteLine("Invalid PDF.");
Tips:
For formats like .docx or .xlsx, check the ZIP signature
and inspect internal files (like [Content_Types].xml) for deeper validation.For larger sets of formats, you can build a dictionary of known signatures.

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How do you validate file format based on file signature (magic number) instead of just file extensio

ICSM Computer

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1 Answers

What is a File Signature / Magic Number?

How to Read and Validate Signature in C#

Usage:

Tips:

Liked By

File Type	Signature (Hex)	ASCII Example
PDF	25 50 44 46	`%PDF`
PNG	89 50 4E 47 0D 0A 1A 0A	`.PNG....`
JPG	FF D8 FF
ZIP	50 4B 03 04	`PK..`
DOCX	50 4B 03 04	ZIP container